In general, the abrasion resistance of steels is correlated with the hardness. For example, for abrasion-resistant steels used in industrial machines such as cutting edges of industrial waste processing machines, a high hardness of from 360 to 550 in Brinell hardness HB on the surface is required. In order to increase the hardness of a steel, it is effective to make the metal structure into martensite by quenching, and conventionally, an abrasion-resistant steel improved in hardenability by containing a variety of alloy elements has been proposed (see, for example, Patent Documents 1 to 4).
In recent years, with the increase in the size of industrial machine and the like, thick abrasion-resistant steels are being required. For example, abrasion-resistant steel plates having a plate thickness of about from 50 to 100 mm are manufactured. From the viewpoint of prolonging the life with respect to abrasion, abrasion-resistant steels having small differences in hardness between surface layers and central portions in the plate thickness are being required. To satisfy such requirements, abrasion-resistant steels containing Nb and B, and further one kind or more of Cu, Ni, Cr, Mo, V, and Ti have been proposed (for example, see Patent Document 5).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2016-79459    Patent Document 2: JP-A No. 2014-194043    Patent Document 3: JP-A No. 2014-194042    Patent Document 4: JP-A No. 2012-214890    Patent Document 5: JP-A No. H09-118950